Variable valve timing control apparatus for internal combustion engine

ABSTRACT

A variable valve timing control apparatus for an engine includes a variable valve timing unit, a lock pin, a hydraulic control unit, and an oil filling control unit. An operation mode of the hydraulic control unit is changed between a lock mode, in which the lock pin locks the camshaft phase at the intermediate lock phase, and a phase feed-back control mode, in which the camshaft phase is controlled to a target phase. The oil filling control unit supplies oil to an advance chamber and a retard chamber of the variable valve timing unit before the operation mode is changed from the lock mode to the phase feed-back control mode after an engine start. The oil filling control unit causes the lock pin to keep locking the camshaft phase until the oil filling control is completed after the engine start.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by reference Japanese Patent Application No. 2008-312843 filed on Dec. 9, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable valve timing control apparatus that adjusts valve timing of an internal combustion engine.

2. Description of Related Art

In a conventional hydraulic variable valve timing unit, as shown in JP-A-H9-324613 (corresponding to U.S. Pat. No. 5,738,056) and JP-A-2001-159330 (corresponding to U.S. Pat. No. 6,330,870), a lock phase during engine stop is set at a generally middle phase within an adjustable range of a camshaft phase such that the adjustable range of valve timing (camshaft phase) is enlarged. In the above conventional art, an intermediate lock phase, at which the phase is locked during the engine stop, is set at a phase suitable for starting the engine. The engine is started while the camshaft phase is at the intermediate lock phase. Also, after starting the engine, the lock is released such that the camshaft phase is shifted toward a target phase that is set in accordance with an engine operational state.

In the variable valve timing unit, oil in the advance chamber (advance-side hydraulic chamber) and in the retard chamber (retard-side hydraulic chamber) are released while the engine stops, and thereby oil pressure in each of the hydraulic chambers is reduced. As a result, if the lock release is executed immediately when the lock release request is issued after the engine start, oil (oil pressure) may not be sufficiently supplied to each hydraulic chamber at the time of issuing. When the lock release is executed in a state, where oil is not sufficiently supplied to each hydraulic chamber (or where each hydraulic chamber has insufficient oil pressure therein), it becomes impossible to maintain the camshaft phase at the moment of unlocking the lock pin. As a result, the wide variation of the camshaft phase, in which the camshaft phase is rapidly changed toward the hydraulic chamber having a lower oil pressure, occurs, and thereby combustion performance may deteriorate, or engine rotation may become unstable disadvantageously.

SUMMARY OF THE INVENTION

The present invention is made in view of the above disadvantages. Thus, it is an objective of the present invention to address at least one of the above disadvantages.

To achieve the objective of the present invention, there is provided a variable valve timing control apparatus for an internal combustion engine that has a camshaft and a crankshaft, the variable valve timing control apparatus including a variable valve timing unit, a lock pin, a hydraulic control unit, and an oil filling control unit. The variable valve timing unit is adapted to adjust valve timing by shifting a camshaft phase that is a rotational angular position of the camshaft relative to a rotational angular position of the crankshaft. The lock pin is adapted to lock the camshaft phase at an intermediate lock phase that is generally middle of an adjustable range of the camshaft phase. The hydraulic control unit is adapted to control oil pressure that actuates the variable valve timing unit and the lock pin, wherein an operation mode of the hydraulic control unit is changed between a lock mode, in which the lock pin locks the camshaft phase at the intermediate lock phase, and a phase feed-back control mode, in which the camshaft phase is controlled to a target phase. The oil filling control unit executes oil filling control for supplying oil to an advance chamber and a retard chamber of the variable valve timing unit before the operation mode is changed from the lock mode to the phase feed-back control mode after a start of the internal combustion engine. The oil filling control unit causes the lock pin to keep locking the camshaft phase by prohibiting the lock pin from releasing the locked camshaft phase until the oil filling control is completed after the start of the internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which:

FIG. 1 is a general schematic configuration of a control system according to the first embodiment of the present invention;

FIG. 2 is a diagram illustrating a variable valve timing unit and an oil pressure control circuit of the first embodiment;

FIG. 3 is a sectional view of the variable valve timing unit of the first embodiment taken along a plane perpendicular to a longitudinal axis of the variable valve timing unit;

FIG. 4 is a timing chart illustrating a control example of the first embodiment, in which example an engine is started while an actual camshaft phase is located at an intermediate lock phase;

FIG. 5 is a timing chart illustrating another control example of the first embodiment, in which example the engine is started while a lock pin is disengaged from a lock hole;

FIG. 6 is a flow chart illustrating a procedure of a start-time variable valve timing control routine in the first embodiment;

FIG. 7 is a diagram illustrating a variable valve timing unit and an oil pressure control circuit of the second embodiment of the present invention;

FIG. 8A is a diagram for explaining a switching pattern for switching an operational state of an advance port, a retard port, and a lock pin control port of a hydraulic control valve of the second embodiment;

FIG. 8B is a control characteristic diagram of the hydraulic control valve for explaining a relation between (a) a phase change speed and (b) four control ranges of a control duty including a lock mode, an advance mode, a hold mode, a retard mode;

FIG. 9 is flow chart illustrating a procedure of a post-start oil filling control routine of the second embodiment;

FIG. 10 is a flow chart illustrating a procedure of a lock-release-request-time oil filling control routine of the second embodiment; and

FIG. 11 is a flow chart illustrating a procedure of an unlocked-time oil filling control routine of the second embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

First and the second embodiments, which applies the present invention to a variable valve timing control apparatus for adjusting an intake valve, will be described below.

The First Embodiment

The first embodiment of the present invention will be described with reference to FIGS. 1 to 6.

As shown in FIG. 1, an engine 11 (internal combustion engine) transmits drive force from a crankshaft 12 to an intake camshaft 16 and an exhaust camshaft 17 through a timing chain 13 and sprockets 14, 15. The intake camshaft 16 is provided with a variable valve timing unit 18 that adjusts an advance amount (or a camshaft phase) of the intake camshaft 16 relative to the crankshaft 12. More specifically, the camshaft phase is a rotational angular position of the intake camshaft 16 relative to a rotational angular position of the crankshaft 12.

Also, a cam angle sensor 19 is provided at a position radially outward of the intake camshaft 16 for outputting cam angle signal pulses at predetermined cam angles in order to identify cylinders. Also, a crank angle sensor 20 is provided at a position radially outward of the crankshaft 12 for outputting crank angle signal pulses at predetermined crank angles. The signals outputted from the cam angle sensor 19 and the crank angle sensor 20 are fed to an engine control circuit 21. The engine control circuit 21 computes actual valve timing (actual camshaft phase) of the intake valve and computes an engine rotation speed based on a frequency (pulse interval) of the output pulses of the crank angle sensor 20. Also, the other signals outputted by various sensors (an intake air pressure sensor 22, a coolant temperature sensor 23, a throttle sensor 24) for detecting an engine operational state are fed to the engine control circuit 21.

The engine control circuit 21 executes fuel injection control and ignition control based on the engine operational state detected by the various sensors. Also, the engine control circuit 21 executes variable valve timing control (phase feed-back control), in which the engine control circuit 21 feed-back controls oil pressure that actuates the variable valve timing unit 18 such that the actual valve timing of the intake valve (or an actual camshaft phase of the intake camshaft 16) becomes target valve timing (target phase).

Next, the variable valve timing unit 18 will be described with reference to FIGS. 2 and 3.

The variable valve timing unit 18 has a housing 31 that is fixed to the sprocket 14 through a bolt 32. The sprocket 14 is movably supported at a position radially outward of the intake camshaft 16. Thus, when the rotation of the crankshaft 12 is transmitted to the sprocket 14 and the housing 31 through the timing chain 13, the sprocket 14 and the housing 31 are rotated synchronously with the crankshaft 12.

The intake camshaft 16 has one end portion that is fixed to a rotor 35 through a bolt 37. The rotor 35 is received within the housing 31 and is rotatable relative to the housing 31.

As shown in FIG. 3, multiple oil filled chambers 40 are formed within the housing 31, and vanes 41 are formed at radially outward parts of the rotor 35. Each of the oil filled chambers 40 is divided into an advance chamber 42 (advance hydraulic chamber) and a retard chamber 43 (retard hydraulic chamber) by the corresponding vane 41. At least one of the vanes 41 has both circumferential ends that are provided with respective stoppers 56. Each of the stoppers 56 limits a rotational range of the rotor 35 (the vane 41) relative to the housing 31. The stoppers 56 defines a full retard phase and a full advance phase of an adjustable range of the camshaft phase.

The variable valve timing unit 18 is provided with an intermediate lock mechanism 50 that is adapted to lock the camshaft phase at an intermediate lock phase. For example, the intermediate lock phase corresponds to a position (phase) that is located generally middle of the above adjustable range. The intermediate lock mechanism 50 will be described below. A lock pin receiving hole 57 is provided to one of the multiple vanes 41, or multiple lock pin receiving holes 57 may be provided to the multiple vanes 41, respectively. The lock pin receiving hole 57 receives therein a lock pin 58 that is displaceable to project from the lock pin receiving hole 57. The lock pin 58 locks the rotation of the rotor 35 (the vane 41) relative to the housing 31 when the lock pin 58 projects from the lock pin receiving hole 57 toward the sprocket 14 to be inserted into a lock hole 59 of the sprocket 14. As a result, the camshaft phase is locked at the intermediate lock phase located generally in the middle of the adjustable range. The intermediate lock phase is set at a position that is suitable for starting the engine 11. It should be noted that the lock hole 59 may be alternatively provided to the housing 31.

The lock pin 58 is urged by a spring 62 in a lock direction for locking the camshaft phase. In other words, the lock pin 58 is urged in a projection direction, in which the lock pin 58 is capable of projecting from the lock pin receiving hole 57. Also, an hydraulic chamber for releasing the lock is formed between (a) the radially outward part of the lock pin 58 and (b) the lock pin receiving hole 57. The hydraulic chamber is used to control oil pressure that actuates the lock pin 58 in a lock release direction for unlocking the rotation of the rotor 35 to release the locked camshaft phase such that the lock pin 58 does not lock the camshaft phase. For example, when the pressure in the hydraulic chamber becomes high, the lock pin 58 is displaced in the lock release direction. Also, when the pressure in the hydraulic chamber becomes low, or is released, the lock pin 58 is urged in the lock direction by the urging force of the spring 62. It should be noted that the housing 31 is provided with a helical torsion spring 55 (see FIG. 2) that provides spring force for assisting oil pressure applied to rotate the rotor 35 relatively in an advance direction during advance control.

In the first embodiment, a hydraulic control unit, which controls oil pressure that actuates the variable valve timing unit 18 and the lock pin 58, includes a phase-control hydraulic control valve 25 (first hydraulic control valve) and a lock-control hydraulic control valve 26 (second hydraulic control valve). The phase-control hydraulic control valve 25 controls oil pressure that actuates the variable valve timing unit 18, and the lock-control hydraulic control valve 26 controls oil pressure that actuates the lock pin 58. For example, the phase-control hydraulic control valve 25 is a five-port and three-position type spool valve, and the lock-control hydraulic control valve 26 is a three-port and two-position type spool valve.

An oil pump 28 is driven by drive force of the engine 11, and the oil pump 28 pumps oil within an oil pan 27 to supply oil to each of hydraulic control valves 25, 26. The phase-control hydraulic control valve 25 controls oil pressure (oil amount) supplied to the advance chambers 42 and the retard chambers 43 of the variable valve timing unit 18. The lock-control hydraulic control valve 26 controls oil pressure (oil amount) that actuates the lock pin 58 in the lock release direction. It should be noted that the phase-control hydraulic control valve 25 is provided with a check valve 29 at an inlet port side of the hydraulic control valve 25, the check valve 29 limiting backflow of oil.

The engine control circuit 21 computes a target phase (target valve timing) based on an engine operational condition during the phase feed-back control (variable valve timing control) in order to feed-back control a control duty (control amount) of the phase-control hydraulic control valve 25 such that an actual camshaft phase of the intake camshaft 16 (actual valve timing of the intake valve) is caused to become a target phase (target valve timing). Thus, pressure of oil supplied to the advance chambers 42 and the retard chambers 43 of the variable valve timing unit 18 is feed-back controlled by the feed back control of the control duty.

When a lock request is issued during the phase feed-back control, a lock control will be executed as follows. For example, in the lock control, the lock pin 58 is caused to be fitted into the lock hole 59 of the sprocket 14 such that the actual camshaft phase is locked at the intermediate lock phase. Firstly, the operational position of the lock-control hydraulic control valve 26 is changed to the drain port for releasing oil pressure in the lock release hydraulic chamber within the lock pin receiving hole 57 such that the spring 62 urges to displace the lock pin 58 in the lock direction. Then, a phase shift control is executed, in which the phase-control hydraulic control valve 25 is controlled such that the actual camshaft phase is shifted in a shift-control direction to a position beyond the intermediate lock phase while the spring 62 urges the lock pin 58 in the lock direction (projection direction). When the actual camshaft phase stops round the intermediate lock phase during the phase shift control, the control duty of the phase-control hydraulic control valve 25 is further changed by a certain amount such that the actual camshaft phase is further shifted in the shift-control direction. In a case, where the actual camshaft phase is not shifted even after the above further change of the control duty, it is determined that the lock has been completed. For example, the completion of the lock indicates that the actual camshaft phase has been locked at the intermediate lock phase by fitting the lock pin 58 into the lock hole 59.

In the first embodiment, in order to reliably execute the lock control of the lock pin 58 and the determination of the completion of the lock, the following control is executed. For example, when the actual camshaft phase stops around the intermediate lock phase during the phase shift control, the control duty of the phase-control hydraulic control valve 25 is changed by a certain amount alternately (a) for shifting the camshaft phase in the advance direction and (b) for shifting the camshaft phase in the retard direction. When the above alternate change of the control duty does not shift the actual camshaft phase in either direction, it is finally determined that the lock has been completed.

Also, in order to start the engine 11, the engine 11 is cranked by a starter (not shown) in a state, where the lock pin 58 locks the actual camshaft phase at the intermediate lock phase. Then, after the starting of the engine, oil filling control for supplying oil to the advance chambers 42 and the retard chambers 43 of the variable valve timing unit 18 is executed before the operation mode is changed from a lock mode (described later) to the phase feed-back control mode. Also, the lock pin 58 is caused to keep locking the actual camshaft phase by prohibiting the lock pin 58 from releasing the locked camshaft phase until the oil filling control is completed after the starting of the engine.

In the above state, the execution of the oil filling control may be continued after the engine start until sufficient supply of oil to each of the hydraulic chambers 42, 43 is detected by using, for example, pressure sensors. However, the employing the oil pressure sensors may lead to cost increase, and thereby failing to satisfy the demand for cost reduction.

Then, in the first embodiment, the execution of the oil filling control is continued until a certain time period has elapsed since engine start (start of the cranking). In other words, the oil filling control is continued for the certain time period after the start of the cranking. A time required for sufficiently supplying each of the hydraulic chambers 42, 43 with oil is measurable in advance based on design data and experimental data, and thereby an execution duration for executing the oil filling control may be preset based on the design data and the experimental data. Thus, it is possible to satisfy the demand for cost reduction by controlling the execution duration of the oil filling control by using a timer, and thereby by simplifying the configuration advantageously.

The first embodiment has a configuration that includes both (a) the phase-control hydraulic control valve 25 for controlling oil pressure that actuates the variable valve timing unit 18 and (b) the lock-control hydraulic control valve 26 for controlling oil pressure that actuates the lock pin 58. Typically, the phase-control hydraulic control valve 25 is operated independently from the lock-control hydraulic control valve 26. Thus, it is possible to control the phase-control hydraulic control valve 25 in the advance direction or in the retard direction while the lock-control hydraulic control valve 26 is controlled in the lock direction.

In consideration of the above configuration, during the execution period of the oil filling control of the first embodiment, while control duty of the lock-control hydraulic control valve 26 is maintained at a value for maintaining the lock pin 58 in a lock position for locking the camshaft phase at the intermediate lock phase, control duty of the phase-control hydraulic control valve 25 is changed such that one of advance chamber oil filling control and retard chamber oil filling control is executed first, and then the other one of the advance chamber oil filling control and the retard chamber oil filling control subsequently. In the advance chamber oil filling control, oil is supplied to the advance chamber 42 such that the advance chamber 42 is filled with oil. Also, in the retard chamber oil filling control, oil is supplied to the retard chamber 43 such that the retard chamber 43 is filled with oil. Due to the above, during the execution period of the oil filling control (the advance chamber oil filling control and the retard chamber oil filling control), the advance chamber 42 and the retard chamber 43 are reliably filled with oil while the lock pin 58 is maintained at the lock position (or while the lock pin 58 keeps locking the camshaft phase at the intermediate lock phase).

Also, in a case, where the disengagement of the lock pin 58 from the lock hole 59 (or unlock of the lock pin 58) is detected during the execution of the oil filling control based on the detection of the shift of the actual camshaft phase, the oil filling control is stopped. Then, control duty of the phase-control hydraulic control valve 25 is changed such that the actual camshaft phase is shifted to the full retard phase (or to the full advance phase). For example, in general, in a case, where the lock pin 58 is unlocked while the advance chambers 42 and the retard chambers 43 are not sufficiently filled with oil, the actual camshaft phase becomes unstable, and thereby the actual camshaft phase may widely vary. Thus, immediately after the detection of that the lock pin 58 does not lock the camshaft phase during the execution of the oil filling control, the oil filling control is stopped and only one of (a) the advance chambers 42 and (b) the retard chambers 43 is filled with oil. As a result, the actual camshaft phase is quickly shifted to the full retard phase (or the full advance phase) within the adjustable range such that the wide unwanted variation of the actual camshaft phase is quickly and successfully prevented.

Furthermore, in the first embodiment, a target idling rotational speed of the internal combustion engine indicates a first value while the lock pin 58 locks the camshaft phase. The target idling rotational speed after the start of the internal combustion engine is increased to be a second value that is higher than the first value when it is detected that the lock pin 58 does not lock the camshaft phase during the execution of the oil filling control. Simultaneously, when it is detected that the lock pin 58 does not lock the camshaft phase during the execution of the oil filling control, the actual camshaft phase is controlled at the full retard phase or at the full advance phase until a certain time period that is shorter than a time required for completing the oil filling control elapses. The operation mode is changed to the normal control (the lock control or the phase feed-back control) after the above operation.

As above, if the target idling rotational speed of the fast idle is increased upon the detection of the unlock of the lock pin 58 during the execution of the oil filling control, a rotational speed (oil discharge rate) of the oil pump 28 is increased such that the advance chamber 42 or the retard chamber 43 is more quickly and effectively filled with oil. As a result, it is possible to suppress the unwanted variation of the actual camshaft phase, and thereby it is possible to effectively prevent the deterioration of combustion and the variation in the rotation of the engine 11 advantageously.

For example, it take a shorter time to sufficiently fulfill only one of the advance chamber 42 and the retard chamber 43 with oil to shift the camshaft phase to the full retard phase or to the full advance phase than to fulfill both of the chambers 42, 43. Thus, even when the unlock of the lock pin 58 is detected during the execution of the oil filling control, the operation is relatively quickly recovered to a normal control while the wide variation of the camshaft phase is limited by firstly shifting the camshaft phase to the full retard phase or the full advance phase before changing the operation to the normal control.

A control example of the variable valve timing unit 18 of the first embodiment at a time of starting the engine will be described with reference to FIG. 4 and FIG. 5.

FIG. 4 is a control example, in which the engine 11 is cranked by a starter in a state, where the lock pin 58 locks the actual camshaft phase at the intermediate lock phase. In the example of FIG. 4, the cranking is started at time t1, and simultaneously the advance chamber oil filling control is executed by changing the control duty of the phase-control hydraulic control valve 25 to an advance-side duty in order to fill the advance chamber 42 with oil. For example, the advance-side duty may be a control duty of 0% and may correspond to a full advance phase. The cranking is ended at time t2, at which the starting of the engine 11 has been completed, and then operation is changed to a normal fast idle control, where the engine rotation speed is controlled at the target idling rotational speed of the fast idle in a normal operation.

The advance chamber oil filling control has been executed for a certain time period B until time t3. Then, at time t3, the retard chamber oil filling control is executed by changing the control duty of the phase-control hydraulic control valve 25 to a retard-side duty in order to fill the retard chamber 43 with oil. For example, the retard-side duty may be a control duty of 100% and may correspond to a full retard phase. The lock release (unlock) of the lock pin 58 is prohibited during a period from the start of the cranking till the end of the advance chamber oil filling control and the retard chamber oil filling control, and thereby the lock pin 58 remains at the lock position, at which the lock pin 58 keeps locking the camshaft phase at the intermediate lock phase.

Then, operation is changed to a normal control (lock control or phase feed-back control) at time t4, by which the retard chamber oil filling control has been executed for a certain time period C. In order to execute the lock control as shown in FIG. 4, the control duty of the phase-control hydraulic control valve 25 is changed to a certain control duty such that the lock pin 58 within the lock hole 59 is displaced in a rotational direction (in the retard direction or in the advance direction) of the rotor 35. Thus, the lock pin 58 is held and is slightly pressed against an inner peripheral edge of the lock hole 59, and thereby the wide variation of the actual camshaft phase is prevented. For example, the above certain control duty may be apart from a hold duty, which is used for maintaining the actual camshaft phase, by a predetermined amount in the retard direction or in the advance direction.

Also, when the lock release request is issued during the execution of the lock control (lock mode), the operational position of the lock-control hydraulic control valve 26 is changed from the drain port to the oil pressure supply port such that oil pressure is supplied to the lock release hydraulic chamber within the lock pin receiving hole 57 in order to actuate the lock pin 58 in the lock release direction for the lock release. In the above, the operation mode is prevented from being changed into the phase feed-back control mode for a certain period after the lock release request is issued. Simultaneously, the control duty of the phase-control hydraulic control valve 25 is controlled such that the actual camshaft phase is controlled around the intermediate lock phase while the lock pin 58 is actuated in the lock release direction. Then, when the certain period has elapsed since the time of issuing the lock release request, operation is changed to the phase feed-back control mode.

As above, the change to the phase feed-back control mode is prohibited for the certain period since the lock release request is issued, and also during the above period, the lock release control is executed while the actual camshaft phase is controlled around the intermediate lock phase, for example, by adjusting the control duty at the certain control duty that is apart from the hold duty by the predetermined amount in the retard direction or in the advance direction. As a result, it is possible to effectively disengage the lock pin 58 from the lock hole 59. More specifically, due to the above operation, while the lock pin 58 is pulled out of the lock hole 59, the lock pin 58 is effectively limited from being urged against the inner peripheral edge of the lock hole 59 by the driving force for shifting the actual camshaft phase by the phase feed-back control. As a result, the possible failure in the disengagement of the lock pin 58 from the lock hole 59 is successfully prevented. Accordingly, after the lock release has been ended, the operation mode is changed into the phase feed-back control mode. Thus, when the lock release request is issued, it is possible to reliably start shifting the actual camshaft phase to the target phase (or it is possible to reliably start the phase feed-back control) after the lock has been released.

In contrast, FIG. 5 is a control example, in which the engine 11 is cranked by the starter for starting the engine 11 in an unlock position, where the lock pin 58 is unlocked. Similar to the other control example in FIG. 4, the control duty of the phase-control hydraulic control valve 25 is also set at the control duty of 0% for the full advance phase in order to start the advance chamber oil filling control at time t1, at which the cranking is started. During the execution of the oil filling control (the advance chamber oil filling control and the retard chamber oil filling control), the unlock of the lock pin 58 is detected based on detection of the change of the camshaft phase at time t2. Then, the advance chamber oil filling control is stopped. Then, the target idling rotational speed of the fast idle executed after the engine start while the lock pin 58 is at the unlock position is set higher than the target idling rotational speed of the normal fast idle, in which the lock pin 58 is at the lock position. Also, simultaneously, the full retard control is executed by changing the control duty of the phase-control hydraulic control valve 25 at the control duty of 100% for the full retard phase for a certain time period such that the actual camshaft phase is shifted to the full retard phase. For example, the certain time period is shorter than a time period required for the execution of the oil filling control (the advance chamber oil filling control and the retard chamber oil filling control). It should be noted that the full advance control may be alternatively executed by changing the control duty of the phase-control hydraulic control valve 25 to the control duty of 0% for the full advance phase such that the actual camshaft phase is shifted to the full advance phase.

When the above certain time period has elapsed at time t3 since the start of the full retard control (or the full advance control), the full retard control (or the full advance control) is ended. By this time, oil pressure has been increased due to elapse of the certain time period, and the operation is changed to the normal control (for example, the phase feed-back control as shown in FIG. 5).

It should be noted that in the examples of FIGS. 4 and 5, the advance chamber oil filling control is firstly executed after the start of the cranking, and then the operation is changed to the retard chamber oil filling control. Alternatively, the retard chamber oil filling control may be firstly executed, and then the operation may be changed to the advance chamber oil filling control.

The variable valve timing control at the engine start according to the first embodiment is executed by the engine control circuit 21 based on a start-time variable valve timing control routine shown in FIG. 6.

The start-time variable valve timing control routine of FIG. 6 is repeatedly executed at certain intervals while the engine control circuit 21 is on (or while the ignition switch is on), and functions as oil filling control means. When the present routine is started, firstly, it is determined at step 101 whether the cranking is being executed or whether a time period that has elapsed since the engine start is shorter than the certain time period A. When the cranking has not been executed yet, or when the certain time period A has already elapsed since the engine start, the start-time variable valve timing control is not required, and thereby the present routine is ended without executing the subsequent process.

In contrast, when it is determined at step 101 that the cranking is being executed or that the elapsed time period since the engine start is shorter than the certain time period A, control proceeds to step 102, where it is determined whether actual camshaft phase varies or not. In other words, it is determined at step 102 whether the actual camshaft phase stably stays around the intermediate lock phase. When it is determined at step 102 that the actual camshaft phase does not vary or that the actual camshaft phase stably stays around the intermediate lock phase, it is determined that the lock pin 58 is at the lock position. Thus, control proceeds to step 104, where it is determined whether an execution duration of the advance chamber oil filling control since the start of the cranking has reached the certain time period B. When it is determined that the execution duration of the advance chamber oil filling control has not reached the certain time period B, control proceeds to step 105, where the control duty of the phase-control hydraulic control valve 25 is maintained at 0% to continue the advance chamber oil filling control for further filling the advance chamber 42 with oil.

Then, the execution duration of the advance chamber oil filling control reaches the certain time period B, control proceeds from step 104 to step 106, where it is determined whether an execution duration of the retard chamber oil filling control has reached a certain time period C. When it is determined that the execution duration of the retard chamber oil filling control has not reached the certain time period C, control proceeds to step 107, where the control duty of the phase-control hydraulic control valve 25 is maintained at 100% in order to continue the retard chamber oil filling control such that the retard chamber 43 is further filled with oil.

When it is determined at step 102 that the lock pin 58 is at the unlock position based on the detection of the change of the actual camshaft phase during the execution of the oil filling control (the advance chamber oil filling control and the retard chamber oil filling control), control proceeds to step 103. At step 103, the oil filling control is stopped, and the control duty of the phase-control hydraulic control valve 25 is changed to 100% (or to 0%). As a result, the full retard control (or the full advance control) is executed such that the actual camshaft phase is maintained at the full retard phase (or the full advance phase). The full retard control (or the full advance control) is executed for the certain time period that is shorter than the time period required for the oil filling control (the advance chamber oil filling control and the retard chamber oil filling control). The oil pressure has been increased due to the above operation, and then the operation is changed to the normal control (the lock control or the phase feed-back control).

In contrast, in a case, where the lock pin 58 has been maintained at the lock position since the start of the cranking, when it is determined at step 106 that the execution duration of the retard chamber oil filling control has reached the certain time period C, the retard chamber oil filling control is ended. Then, control proceeds to step 108, where it is determined whether the lock request is issued. When the lock request is issued, control proceeds to step 109, where the control duty of the phase-control hydraulic control valve 25 is changed to the certain control duty such that the lock pin 58 is displaced within the lock hole 59 in the retard direction or in the advance direction in order to maintain the lock pin 58 in a state (lock mode), where the lock pin 58 is slightly pressed against the inner peripheral edge of the lock hole 59. For example, the above certain control duty may be a control duty apart from the hold duty, which is used for maintaining the actual camshaft phase, by the predetermined amount rx in the retard direction or in the advance direction.

Then, when the lock release request is issued, control proceeds from step 108 to step 110, where the lock release control is executed. More specifically, in the lock release control, the operational position of the lock-control hydraulic control valve 26 is changed from the drain port to the oil pressure supply port such that oil pressure is supplied to the lock release hydraulic chamber within the lock pin receiving hole 57 in order to actuate the lock pin 58 in the lock release direction for the lock release. Then, control proceeds to step 111, where the operation is changed to the phase feed-back control. In the above case, even when the lock release request is issued before completing the oil filling control (the advance chamber oil filling control and the retard chamber oil filling control), the lock release is prohibited until the completion of the oil filling control, and the lock release control is executed after the completion of the oil filling control.

In the first embodiment, before the operation is changed from the lock mode to the phase feed-back control mode after the engine start, the oil filling control, which supply oil to the advance chambers 42 and the retard chambers 43 of the variable valve timing unit 18, is executed. Also, simultaneously, the lock release of the lock pin 58 is prohibited until the end of the oil filling control after the engine start such that the lock pin 58 is maintained at the lock position. Thus, even when the lock release request is issued before the completion of the oil filling control or before fulfillment of the advance chambers 42 and the retard chambers 43 with oil, the execution of the lock release is prevented, and thereby the wide and unstable variation of the camshaft phase at the time of the lock release is effectively prevented.

Second Embodiment

The above the first embodiment illustrates a configuration that has the phase-control hydraulic control valve 25 and the lock-control hydraulic control valve 26, which are independent from each other. More specifically, the phase-control hydraulic control valve 25 independently controls oil pressure that actuates the variable valve timing unit 18, and the lock-control hydraulic control valve 26 independently controls oil pressure that actuates the lock pin 58. However, in the second embodiment shown in FIGS. 7 to 11, a hydraulic control valve 71 is singularly employed instead. More specifically, the hydraulic control valve 71 integrally has both (a) phase-control hydraulic control valve function for controlling oil pressure that actuates a variable valve timing unit 70 and (b) lock-control hydraulic control valve function for controlling oil pressure that actuates the lock pin 58.

A configuration of the variable valve timing unit 70 of the second embodiment is substantially similar to a configuration of the variable valve timing unit 18 of the first embodiment. Thus, the numerals same with the first embodiment are used in the description below.

As above, the hydraulic control valve 71 integrally has the phase-control hydraulic control valve function and the lock-control hydraulic control valve function, and is, for example, an eight-port and four-position spool valve. As shown in FIGS. 8A and 8B, the operation mode of the hydraulic control valve 71 is categorized in four modes in accordance with the control duty of the hydraulic control valve 71. For example, the four modes include a lock mode (slight advance mode), an advance mode, a hold mode, and a retard mode.

When the operation mode is in the lock mode (slight advance mode), a lock pin control port of the hydraulic control valve 71 is brought into communication with the drain port such that oil pressure in the lock release hydraulic chamber within the lock pin receiving hole 57 is released, and thereby the spring 62 urges the lock pin 58 in the lock direction (projection direction). Also, a retard port of the hydraulic control valve 71 is brought into communication with the drain port such that oil pressure in the retard chambers 43 are released. In the above communication state, a restrictor in an oil passage connected with an advance port of the hydraulic control valve 71 is slowly changed in accordance with the control duty of the hydraulic control valve 71 such that oil is slowly supplied to the advance chambers 42 through the advance port. As a result, the actual camshaft phase is gently shifted in the advance direction.

When the operation mode is in the advance mode, the retard port of the hydraulic control valve 71 is brought into communication with the drain port such that oil pressure in the retard chamber 43 is released. In the above operation state, oil pressure supplied to the advance chambers 42 through the advance port of the hydraulic control valve 71 is changed in accordance with the control duty of the hydraulic control valve 71. As a result, the actual camshaft phase is shifted in the advance direction.

When the operation mode is in the hold mode, oil pressure in both the advance chamber 42 and the retard chamber 43 are maintained such that the actual camshaft phase is prevented from shifting.

When the operation mode is in the retard mode, the advance port of the hydraulic control valve 71 is brought into communication with the drain port such that oil pressure in the advance chambers 42 are released. In the above operation state, oil pressure supplied to the retard chambers 43 through the retard port of the hydraulic control valve 71 is changed in accordance with the control duty of the hydraulic control valve 71 such that the actual camshaft phase is shifted in the retard direction.

When the operation mode is in the control mode other than the lock mode (such as the retard mode, the hold mode, the advance mode), the lock release hydraulic chamber within the lock pin receiving hole 57 is filled with oil in order to increase oil pressure in the lock release hydraulic chamber. As a result, the increased pressure of oil pulls the lock pin 58 out of the lock hole 59 such that the lock of the lock pin 58 is released. In other words, the increased oil pressure disengages the lock pin 58 from the lock hole 59 such that the lock of the camshaft phase by the lock pin 58 is released.

It should be noted that in the second embodiment, the control mode is changed in the order from the lock mode (slight advance mode), the advance mode, the hold mode, to the retard mode in accordance of the increase of the control duty of the hydraulic control valve 71. However, for example, the control mode may be alternatively changed in the order of the retard mode, the hold mode, the advance mode, and the lock mode (slight advance mode) in accordance with the increased of the control duty of the hydraulic control valve 71 as shown in FIG. 8B. Further alternatively, the control mode may be changed in the other order of the lock mode (slight advance mode), the retard mode, the hold mode, and the advance mode. Also, in a case, where a control range of the lock mode is directly adjacent to a control range of the retard mode, the operation of the hydraulic control valve 71 in the control range for the lock mode (slight retard mode) may be executed as follows. For example, in the lock mode, oil pressure in the lock release hydraulic chamber within the lock pin receiving hole 57 is released, and the spring 62 is caused to urge the lock pin 58 in the lock direction. Simultaneously, the advance port is brought into communication with the drain port such that oil pressure is the advance chamber 42 is released. In the above operation condition, an operational state of the restrictor of the oil passage connected with the retard port is slowly changed in accordance with the control duty of the hydraulic control valve 71 such that oil is slowly supplied to the retard chambers 43 through the retard port. As a result, the actual camshaft phase is gently shifted in the retard direction.

The engine control circuit 21 computes the target phase (target valve timing) based on the engine operational condition during the phase feed-back control (variable valve timing control). Then, the control duty of the hydraulic control valve 71 is feed-back controlled such that oil pressure supplied to the advance chamber 42 and the retard chambers 43 of the variable valve timing unit 70 is feed-back controlled in order to cause the actual camshaft phase of the intake camshaft 16 (actual valve timing of the intake valve) to become the target phase (target valve timing).

When the lock request is issued during the phase feed-back control, the phase shift control is executed, in which the hydraulic control valve 71 is controlled such that the actual camshaft phase is shifted to go beyond the intermediate lock phase in the direction (for example, in the retard direction in the second embodiment) that is opposite from the shift-control direction during the lock mode to come. Then, the operation mode is changed to the lock mode, in which the hydraulic control valve 71 is controlled such that the actual camshaft phase is gently shifted in the direction toward the intermediate lock phase (or in the advance direction in the second embodiment) while the spring 62 is caused to urge the lock pin 58 in the lock direction.

Furthermore, in the second embodiment, the hydraulic control valve 71 is configured such that phase shift torque in the advance direction (or in the retard direction) is adjustable in accordance with the control duty within the control range of the lock mode. When the operation mode is changed to the lock mode, the spring 62 urges the lock pin 58 in the lock direction and the actual camshaft phase is gently shifted in the direction toward the intermediate lock phase. During the above lock mode, the hydraulic control valve 71 is controlled such that the phase shift torque is gradually increased. When the actual camshaft phase stops around the intermediate lock phase even with the gradual increase of the phase shift torque, it is determined that the lock has been completed.

Present embodiment employs the hydraulic control valve 71 that integrally includes the phase-control hydraulic control valve function and the lock-control hydraulic control valve function as described above. Even in a case, where the actual camshaft phase accidentally stops at a position different from the intermediate lock phase due to some reasons (for example, condition, such as oil temperature) in the present embodiment, the above configuration and operation of the hydraulic control valve 71 is capable of effectively preventing the erroneous determination of the completion of the lock. As a result, the actual camshaft phase is reliably locked at the intermediate lock phase, and thereby the determination of the completion of the lock is reliably achieved.

The control characteristic of FIGS. 8A and 8B shows that the lock mode is not prepared for the retard mode, and that the control range of the lock mode only ranges beside the advance mode. As a result, it is impossible to supply oil to the retard chamber 43 while the lock pin 58 is maintained at the lock position.

Thus, in the second embodiment, in a case, where the oil filling control is executed after the engine start, the control duty of the hydraulic control valve 71 is changed such that the advance chamber oil filling control is firstly executed while the lock pin 58 is maintained at the lock position to keep locking the actual camshaft phase at the intermediate lock phase. When a first lock release request after the engine start is issued, the control duty of the hydraulic control valve 71 is changed such that the retard chamber oil filling control is subsequently executed until the actual camshaft phase starts shifting, or until a certain time period elapses. For example, in the advance chamber oil filling control, it is possible to supply oil to one of the advance chamber 42 and the retard chamber 43 while the lock pin 58 locks the camshaft phase. The one of the hydraulic chambers corresponds to the advance chamber 42 for the case of the control characteristic of FIGS. 8A and 8B. Also, in the retard chamber oil filling control, oil is supplied to the other one of the hydraulic chambers, and the other one corresponds to the retard chamber 43 based on the control characteristic of FIGS. 8A and 8B.

In the above case, during the execution of the advance chamber oil filling control, when the unlock of the lock pin 58 is detected based on the detection of the change of the actual camshaft phase, oil is supplied to the other one (the retard chamber 43) of the advance chamber 42 and the retard chamber 43. Then, operation is changed to the normal control (the lock control or the phase feed-back control). Due to the above operation, even when the lock pin 58 is erroneously unlocked (or the lock pin 58 is erroneously displaced to the unlock position) during the execution of the oil filling control, the operation is relatively early changed to the normal control while the wide variation of the actual camshaft phase is effectively prevented.

The oil filling control of the second embodiment is executed by the engine control circuit 21 based on each of routines shown in FIGS. 9 to 11. Procedure of each of the routines will be described below.

[Post-Start Oil Filling Control Routine]

A post-start oil filling control routine of FIG. 9 is repeatedly executed at certain intervals while the engine control circuit 21 is on, and functions as oil filling control means. When the present routine is started, firstly, it is determined at step 201 whether the cranking is being executed or whether a time period that has elapsed since the start of the cranking is shorter than a certain time period D. When it is determined that the cranking has not been started or that the certain time period D has elapsed since the start of the cranking (since the engine start), corresponding to NO at step 201, the oil filling control is not required, and thereby the present routine is ended without executing the subsequent process.

In contrast, when it is determined at step 201 that the cranking is being executed or that the certain time period D has not elapsed since the engine start, control proceeds to step 202, where it is determined whether the lock pin 58 is unlocked (the lock pin 58 is at the unlock position) based on the change of the actual camshaft phase. When it is determined at step 202 that the lock pin 58 is at an unlock position (the lock pin 58 does not lock the camshaft phase), control proceeds to step 203, where a unlocked-time oil filling control routine of FIG. 11 (described later) is executed.

When it is determined at step 202 that the lock pin 58 is not at the unlock position (or that the actual camshaft phase stably stays around the intermediate lock phase), control proceeds to step 204, where the control duty of the hydraulic control valve 71 is set at a control duty E (see FIG. 8B), at which advance torque is maximized while the control duty is within a control range for the lock mode for maintaining the lock pin 58 at the lock position. For example, when the advance torque is maximized, an oil filling rate for supplying oil to the advance chamber 42 is maximized. As above, the advance chamber oil filling control for supplying oil to the advance chamber 42 is executed. In the present embodiment, it is possible to supply oil to the advance chamber 42 while the lock pin 58 is maintained at the lock position for locking the actual camshaft phase.

Then, control proceeds to step 205, where it is determined whether the actual camshaft phase stably stays around the intermediate lock phase. When it is determined that the actual camshaft phase does not stably stay around the intermediate lock phase, control proceeds to step 206, where the unlock of the lock pin 58 is determined (or it is determined that the lock pin 58 is at the unlock position). Thus, control proceeds to step 203, where the unlocked-time oil filling control routine of FIG. 11 is executed.

In contrast, when it is determined at step 205 that the actual camshaft phase stably stays around the intermediate lock phase, control proceeds to step 207, where the execution duration of the lock mode (post-start oil filling control) exceeds a certain time period F. When it is determined that the execution duration of the lock mode has not exceeded the certain time period F, the present routine is ended.

Then, when the execution duration of the lock mode exceeds the certain time period F, it is determined that supply of oil to the advance chamber 42 has been completed, and control proceeds to step 208, where the control duty of the hydraulic control valve 71 is set at a control duty of 0% within the control range for the lock mode in order to stop the supply of oil to the advance chamber 42. Then, the advance chamber oil filling control is ended, and control proceeds to step 209, where an advance chamber oil filling control completion flag is set at “ON” indicating the completion of the advance chamber oil filling control.

[Lock-Release-Request-Time Oil Filling Control Routine]

The lock-release-request-time oil filling control routine of FIG. 10 is repeatedly executed at certain intervals while the engine control circuit 21 is on, and functions as oil filling control means. When the present routine is started, firstly, it is determined at step 301 whether the advance chamber oil filling control completion flag is ON indicating that the advance chamber oil filling control is completed. When the advance chamber oil filling control completion flag is OFF indicating that the advance chamber oil filling control has not been completed yet, process is ended without executing the subsequent process. Thus, even when the lock release request is issued before the advance chamber oil filling control is completed, the execution of the lock release is prevented.

In contrast, when it is determined at step 301 that the advance chamber oil filling control completion flag is ON indicating that the advance chamber oil filling control has been completed, control proceeds to step 302, where it is determined whether a first lock release request after the engine start is issued. When the first lock release request after the engine start is not issued, the present routine is ended without executing the subsequent process.

Then, when the first lock release request after the engine start is issued, control proceeds to step 303, where the control duty of the hydraulic control valve 71 is set at the control duty of 100% in order to execute the retard chamber oil filling control for supplying oil to the retard chamber 43. For example, when the control duty of the hydraulic control valve 71 is set at the control duty of 100%, the oil filling rate for supplying oil to the retard chamber 43, which has not yet been sufficiently filled with oil, is maximized.

Then, control proceeds to step 304, where a retard chamber oil filling duration counter is incremented, and a retard chamber oil filling duration (or duration time of the control duty 0%) is measured. Then, control proceeds to step 305, where it is determined whether the actual camshaft phase starts shifting. When it is determined that the actual camshaft phase has not started shifting, control proceeds to step 307, where it is determined whether a count value of the retard chamber oil filling duration counter exceeds a predetermined value (or whether a duration for executing the retard chamber oil filling control exceeds a certain time period). When it is determined that the count value of the retard chamber oil filling duration counter is equal to or smaller than the predetermined value, the present routine is ended. Thus, when the first lock release request after the engine start is issued after the execution of the advance chamber oil filling control, the retard chamber oil filling control is executed until the actual camshaft phase starts shifting or until the certain time period has elapsed.

When the actual camshaft phase does not start shifting, and also the retard chamber oil filling duration exceeds the certain time period, the retard chamber oil filling control is ended. Then, control proceeds to step 308, where the lock release control is executed to release the lock of the lock pin 58. The above lock release control may be executed while the target phase of the actual camshaft phase is set around the intermediate lock phase such that the lock pin 58 is limited from being firmly pressed against the inner peripheral edge of the lock hole 59. As a result, frictional force generated while the lock pin 58 is being pulled out of the lock hole 59 during the lock release control is reduced. As above, the lock release control is executed while the control duty of the hydraulic control valve 71 is held within the control range for the hold mode. After the execution of the lock release control, control proceeds to step 309, where the phase feed-back control is executed such that the control duty of the hydraulic control valve 71 is feed-back controlled in order to make the actual camshaft phase become the target phase. As a result, oil pressure of the variable valve timing unit 70 supplied to the advance chamber 42 and the retard chamber 43 is feed-back controlled accordingly.

in contrast, when the actual camshaft phase starts moving before the retard chamber oil filling duration reaches the certain time period, it is determined that the lock release has been already made. Thus, control proceeds to step 306, where the phase feed-back control is executed.

[Unlocked-Time Oil Filling Control Routine]

The unlocked-time oil filling control routine of FIG. 11 is a subroutine executed at step 203 of the post-start oil filling control routine of FIG. 9 when the unlock of the lock pin 58 is detected (or when it is detected that the lock pin 58 does not lock the camshaft phase) at step 202 while the cranking is being executed or before the certain time period D has elapsed since the engine start (step 201).

When the present routine is started, firstly at step 401, the control duty of the hydraulic control valve 71 is set at the control duty of 100%, at which the oil filling rate for the retard chamber 43 is maximized and the retard chamber oil filling control for supplying oil to the retard chamber 43 is executed. Then, control proceeds to step 402, where it is determined whether the engine is idling. When it is determined that the engine is idling, control proceeds to step 403, where the target idling rotational speed is increased such that the engine rotation speed is increased accordingly. Thus, the rotational speed of the oil pump 28 that pumps oil is increased, and thereby the oil filling rate for supplying oil to the retard chamber 43 is increased. As a result, oil pressure in the retard chamber 43 is quickly increased.

When it is determined at step 402 that the engine is not idling, the increase of the target idling rotational speed at step 403 is not executed.

Then, control proceeds to step 404, where it is determined whether the retard chamber oil filling duration (or the duration time of the control duty 100%) exceeds a certain time period G. When it is determined that the retard chamber oil filling duration has not exceeded the certain time period G, the present routine is ended.

When the retard chamber oil filling duration exceeds the certain time period G, it is determined that the supply (filling) of oil to the retard chamber 43 has been completed. Thus, control proceeds to step 405, where the operation is changed to the normal control (the lock control or the phase feed-back control).

In the second embodiment, the variable valve timing control apparatus employs the hydraulic control valve 71 that integrally has (a) the phase-control hydraulic control valve function for controlling oil pressure that actuates the variable valve timing unit 70 and (b) the lock-control hydraulic control valve function for controlling oil pressure that actuates the lock pin 58. When the oil filling control is executed after the engine start, the control duty of the hydraulic control valve 71 is changed such that the advance chamber oil filling control is executed firstly, and such that when the first lock release request after the engine start is issued after the execution of the advance chamber oil filling control, the retard chamber oil filling control is subsequently executed until the actual camshaft phase starts shifting or until the certain time period elapses. For example, in the advance chamber oil filling control, oil is supplied to one of the advance chamber 42 and the retard chamber 43, and it is possible to supply oil to the one of the chambers 42, 43 while the lock pin 58 is kept at the lock position. In accordance with the control characteristic of FIGS. 8A and 8B, the one of the chambers 42, 43 corresponds to the advance chamber 42. In contrast, in the retard chamber oil filling control, oil is supplied to the other one of the chambers, and the other one of the chambers corresponds to the retard chamber 43 based on the control characteristic of FIGS. 8A and 8B. As a result, even in a case for employing the hydraulic control valve 71 that integrally has the phase-control hydraulic control valve function and the lock-control hydraulic control valve function, the unstable wide variation of the camshaft phase at the time of the lock release after the engine start is effectively prevented.

Furthermore, in the second embodiment, when the unlock of the lock pin 58 is detected based on the detection of the change of the actual camshaft phase during the execution of the oil filling control to the advance chamber 42, firstly, oil is supplied to the retard chamber 43. Then, the operation is changed to the normal control (the lock control or the phase feed-back control). As a result, even when the lock pin 58 is unlocked during the execution of the oil filling control to the advance chamber 42, it is possible to relatively early change the operation to the normal control while the unstable wide variation of the actual camshaft phase is effectively prevented.

It should be noted that the present invention is not limited to the variable valve timing control apparatus for the intake valve. However, the present invention may be applied to a variable valve timing control apparatus for an exhaust valve.

A configuration of the variable valve timing unit 18, 70 and a configuration of the hydraulic control valves 25, 26, 71 may be modified as required provided that the modification does not deviate from the gist of the invention.

Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. 

1. A variable valve timing control apparatus for an internal combustion engine that has a camshaft and a crankshaft, comprising: a variable valve timing unit adapted to adjust valve timing by shifting a camshaft phase that is a rotational angular position of the camshaft relative to a rotational angular position of the crankshaft; a lock pin adapted to lock the camshaft phase at an intermediate lock phase that is generally middle of an adjustable range of the camshaft phase; a hydraulic control unit adapted to control oil pressure that actuates the variable valve timing unit and the lock pin, wherein an operation mode of the hydraulic control unit is changed between a lock mode, in which the lock pin locks the camshaft phase at the intermediate lock phase, and a phase feed-back control mode, in which the camshaft phase is controlled to a target phase; and oil filling control means for executing oil filling control for supplying oil to an advance chamber and a retard chamber of the variable valve timing unit before the operation mode is changed from the lock mode to the phase feed-back control mode after a start of the internal combustion engine, wherein: the oil filling control means causes the lock pin to keep locking the camshaft phase by prohibiting the lock pin from releasing the locked camshaft phase until the oil filling control is completed after the start of the internal combustion engine.
 2. The variable valve timing control apparatus according to claim 1, wherein: the oil filling control means executes the oil filling control until a certain time period elapses since the start of the internal combustion engine.
 3. The variable valve timing control apparatus according to claim 1, wherein: the hydraulic control unit includes a first hydraulic control valve and a second hydraulic control valve, the first hydraulic control valve controlling oil pressure for actuating the variable valve timing unit, the second hydraulic control valve controlling oil pressure for actuating the lock pin; the oil filling control means maintains a control amount of the second hydraulic control valve at a value for causing the lock pin to keep locking the camshaft phase during an execution period for executing the oil filling control; and the oil filling control means also changes a control amount of the first hydraulic control valve during the execution period in order to firstly execute one of (a) advance chamber oil filling control for supplying oil to the advance chamber and (b) retard chamber oil filling control for supplying oil to the retard chamber and in order to subsequently execute the other one of (a) the advance chamber oil filling control and (b) the retard chamber oil filling control.
 4. The variable valve timing control apparatus according to claim 1, wherein: the oil filling control means stops the oil filling control and changes a control amount of the hydraulic control unit such that the camshaft phase is controlled to a full retard phase or a full advance phase when it is detected that the lock pin does not lock the camshaft phase based on detection of change of the camshaft phase during the execution of the oil filling control.
 5. The variable valve timing control apparatus according to claim 4, wherein: a target idling rotational speed of the internal combustion engine indicates a first value while the lock pin locks the camshaft phase; and the oil filling control means includes means for increasing the target idling rotational speed after the start of the internal combustion engine to be a second value that is higher than the first value when it is detected that the lock pin does not lock the camshaft phase during the execution of the oil filling control.
 6. The variable valve timing control apparatus according to claim 4, wherein: when it is detected that the lock pin does not lock the camshaft phase during the execution of the oil filling control, the oil filling control means executes the followings: the oil filling control means stops the oil filling control; and the operation mode is changed to a normal control after the oil filling control means controls the camshaft phase at a full retard phase or at a full advance phase until a certain time period that is shorter than a time required for completing the oil filling control elapses.
 7. The variable valve timing control apparatus according to claim 1, wherein: the hydraulic control unit employs a hydraulic control valve that controls oil pressure for actuating the variable valve timing unit and the lock pin: the oil filling control means changes a control amount of the hydraulic control valve such that oil is firstly supplied to one of the advance chamber and the retard chamber while the lock pin locks the camshaft phase after the start of the internal combustion engine; and the oil filling control means changes the control amount of the hydraulic control valve such that oil is subsequently supplied to the other one of the advance chamber and the retard chamber until a certain time period elapses or until the camshaft phase starts shifting when a first lock release request since the engine start is issued.
 8. The variable valve timing control apparatus according to claim 7, wherein: when it is detected that the lock pin does not lock the camshaft phase during the execution of the oil filling control, the operation mode is changed to a normal control after the oil filling control means supplies oil to the other one of the advance chamber and the retard chamber. 